Speed matters a lot!

Amazon: 100 ms of extra load time caused a 1% drop in sales (source: Greg Linden, Amazon).

Google: 500 ms of extra load time caused 20% fewer searches (source: Marrissa Mayer, Google).

Yahoo!: 400 ms of extra load time caused a 5–9% increase in the number of people who clicked “back” before the page even loaded (source: Nicole Sullivan, Yahoo!).